A. Field
The invention relates to a process for the solid phase continuous polymerisation of polyester. More exactly, the invention relates to a process for the solid phase continuous polymerisation of polyester in order to increase its molecular weight.
B. Related Art
It is known that the molecular weight of a polyester can be measured by the measure of its intrinsic viscosity IV (“Intrinsic Viscosity”).
It is also known that the molecular weight increase of a polyester can be achieved by subjecting low molecular weight polyesters, generally in granules or chips form, to a solid phase polymerisation process that can be carried out in a continuous moving-bed or in a static-bed (so called because the polymer bed is not fluidised).
Moving-bed or static-bed solid-phase polymerisation processes, particularly intended for the polyethylene terephthalate, wherein temperatures comprised in the range 180-245° C. are applied, are known, for instance, from U.S. Pat. No. 3,405,098, U.S. Pat. No. 4,064,112, U.S. Pat. No. 4,161,578, U.S. Pat. No. 4,223,128, U.S. Pat. No. 4,238,593, U.S. Pat. No. 5,408,035, U.S. Pat. No. 5,536,810, U.S. Pat. No. 5,590,479, U.S. Pat. No. 5,708,124 and EP 0,222,714.
According to the teaching of the above mentioned documents, the solid phase polymerisation is preceded by a crystallisation step that can be performed at a lower temperature (see, for instance, U.S. Pat. No. 3,405,098, U.S. Pat. No. 4,161,578 and U.S. Pat. No. 4,223,128), at the same temperature (see, for instance, EP 222,714) or at a higher temperature (see, for instance, U.S. Pat. No. 4,064,112) with respect to that applied in the following polymerisation thermal treatment.
The purpose of the crystallisation step prior to the solid phase polymerisation is to prevent the sticking of the granules during the polymerisation process, especially at the highest temperatures.
As a matter of fact, it is known that in industrial solid-phase polymerisation plants sticking phenomena and solid agglomeration of the polyester granules happen frequently.
This problem is particularly evident when the polyester used as raw material in the polymerisation plant is substituted with a different polyester having different needs for the molecular weight increase. This happens for example during solid phase continuous moving-bed polymerisation in producing PET for beverage bottles where polymerisation is carried out at temperatures above the amorphous polyester (prepolymer) glass transition temperature, but below the melting point.
If we analyse all conventional solid phase polymerisation processes available today, it will result that the polyester prepolymer (crystallized or partially crystallized) is fed into the top of a vertical moving or static bed reactor for solid phase polymerisation in which it moves down by gravity in contact with a stream of preheated purge gas.
According to known prior art, the purge gas primarily functions to carry off unwanted by-products such as glycols, water and acetaldehyde, which are generated during polymerisation, while the polyester gradually moves towards the bottom of the vertical reactor.
In general, there are three important requisites that are to be met for correct operation of a continuous solid phase polymerisation process.
First, a steady uninterrupted flow of polymer granules must be maintained. As a consequence, it is highly important that agglomeration or sticking of polymer granules be avoided because they would impede the smooth flow of granules and make discharge of the product from the reactor difficult, thereby causing the plant control losing.
Secondly, a suitable combination of reactor residence time and temperature of granules is required to achieve the desired molecular weight, which is measurable, as indicated above, in terms of its intrinsic viscosity (“IV”). Since reaction rate increases with increasing temperature, and IV increases with increasing residence time, desired IV can be attained either by using relatively long residence time with relatively low temperature or relatively short residence time with relatively high temperature. However, the ideal combination of reactor residence time and temperature must be chosen taking into account the first of the requisites indicated above, i.e. the need to maintain a constant flow of polyester granules, thereby avoiding lumping or sticking of granules.
Third, the flow regime of polyester granules under processing inside solid-phase polymerisation reactor, must be as close as possible to the ideal “plug flow” behaviour, in a way that all polyester granules passing through the reactor experience the same process conditions for the same time duration, giving as a consequence narrow molecular weight distribution in the obtained product, and more generally narrow distribution of polymerised granules final attributes, which is a key factor for the correct realisation of the following steps in processing product with increased molecular weight.
As regards the first requisite, that is the need to avoid the sticking of the polyester granules, it is to be said that this phenomenon is mainly affected by temperature, granules size, bed height, velocity of flow of granules within the reactor and polyester-morphology.
The polyester granules, initially moving freely in a moving bed can stick and clot if, for instance, temperature or bed height increase or if rate decreases.
At solid phase polymerisation conditions, polyester is only partially crystalline (typically with 25 to 65% crystallinity). As a consequence, such polyester is not a rigid body, but rather, it is leathery and slightly tacky.
Since tackiness of polymer increases with increasing temperature, the sticking tendency of polyester granules also increases with increasing temperature.
Consider a fixed bed of polyester granules held motionless inside a solid state polymerisation vertical, cylindrical reactor: on these conditions at polymerisation temperatures and under pressure due to the weight of the polyester bed, granules to be polymerised, creep into one another at contact points and, in time, polymer granules will tend to agglomerate and form larger lumps.
The most effective way to prevent lumping is to constantly renew the inter-granular contact areas so that polymer granules do not have a chance to creep into one another. This is done by maintaining constant flow of polymer granules at sufficiently high velocity.
Since sticking tendency increases with increasing specific surface area (area per unit mass) or, more precisely, with increasing specific contact area of polymer granules, it also increases with decreasing size of polymer granules.
A reduced granules size contributes to accelerate the polymerisation process, on the other hand, however this increases the sticking tendency of polymer granules. In the presence of small size granules it is therefore required to counteract the higher sticking tendency with a reduction in temperature, which, on the other hand, brings the final values of the process rate back to the typical ones for larger size granules processed at a higher temperature.
Furthermore, if the particle size is reduced below certain limits, agglomeration occurs practically at any temperature. Typically that suitable size of polymer granules for solid state polymerisation is between 0.015 to 0.055 grams per granule.
Within a static or moving bed, the compaction pressure a polymer granule can experience is approximately proportional to the weight of the polymer granules in the bed which, in turn, is proportional to the bed height above the granules. Therefore polymer sticking tendency is highest at the bottom of the bed and lowest at the top. As a result, lumping of polymer granules usually starts near the bottom of the bed. For this reason there is a practical limit on the bed height of a solid phase polymerisation reactor. At sufficiently high flow velocity, polymer granules change their positions relative to each other (by sliding, for example), and are thereby prevented from forming lumps. Since the rate of changes of contact areas of polymer granules and the reduction in the bulk density of the bed increases with increasing granule velocity, polymer sticking tendency within the reactor decreases with increasing granule velocity. For every combination of reactor temperature, bed height, and particle size, there exists a minimum granule velocity necessary to prevent sticking. For any given size and shape of polymer granule, the minimum velocity for avoiding sticking increases with increasing temperature and bed height. Thus a higher velocity is required for a higher polymerisation temperature or greater bed height.
For instance, for a pilot scale moving-bed vertical cylindrical reactor according to the known prior art, which is usually no more than 5 meters high, granule velocity of less than 0.3 meter per hour can be used without polymer sticking. On the other hand, for commercial scale vertical reactors, with output for instance up to 300 metric ton/day and which are conventionally 18 to 22 meters high, a granule velocity of at least 2 meters per hour is generally required.
A well designed commercial scale solid phase polymerisation plant must be capable of continuously producing products of desired IV in compliance with the required specification at a sufficiently high throughput.
The currently used plants (i.e. Buehler, UOP-Sinco, Hosokawa-Bepex, Zimmer) use single or multiple vertical cylindrical reactors 10 to 30 meters in height. In those plants the reactor is operated at a temperature between 200° C. and 230° C. and a polyester granules moving velocity of 1.00 to 2.52 meters per hour. Within these ranges of temperature, bed height, and granule velocity, a most suitable combination of the three variables is chosen to produce product with the desired IV. Said conventional plants, today available, are capable of producing polyethylene terephthalate (PET) resin with an IV between 0.72 to 0.86 dl/g, using PET prepolymer with an IV between 0.55 to 0.65 dl/g. Said conventional plants can increase polymer IV by about 0.12 to 0.25 dl/g.
For some specific applications, e.g., PET with IV between 0.95 and 1.05 dl/g for manufacturing technical/commercial articles (luggage, cords, conveyor belts, etc.) or for tyre cords using PET prepolymer with a typical IV in the range between 0.55 and 0.65 dl/g, or for standard bottle applications where the initial IV of the prepolymer is 0.25-0.45 dl/g, it is however, necessary to increase IV by more than 0.25 dl/g. This can hardly be achieved and it often cannot be achieved in a conventional plant using vertical reactors.
In a conventional process, there are two ways to raise the product IV; namely, increasing the reactor temperature or increasing the reactor residence time of granules. The reactor residence time is constrained by bed height and granule velocity. It can be increased by either increasing the bed height or by decreasing granule velocity. Increasing the reactor diameter allows an increase in the throughput rate but not in residence time at constant granule velocity. On the other hand, if reactor temperature is raised to increase the end product IV, polymer sticking tendency will therefore increase. To prevent polymer sticking, bed height must be decreased or granule velocity increased. However, either measure reduces reactor residence time and offsets the effect of the temperature increase. Alternatively, increasing the reactor residence time either by increasing the bed height (assuming there is a sufficient reactor height) or by reducing the granule velocity results in increased polymer sticking tendency.
To prevent sticking, the reactor temperature must be lowered, again offsetting the effect of the increased residence time on the product IV.
These constraints limit the ability of conventional plants using vertical single or multiple reactors, to increase intrinsic polymer IV.
Similar situation it is encountered when an industrial scale plant with capacity above 360 metric tons per day has to be designed for conventional continuous solid phase polymerisation processes.
In fact, in a conventional process, there are two ways to reach high plant production capacity: again by increasing the reactor temperature or by increasing the product volume (“hold-up”) in the reactor. As far as drawbacks due to the temperature increase are concerned, the same above described issues have to be considered. On the other hand, the product volume (“hold-up”) of polyester granules in the reactor is constrained by bed height, reactor diameter and granule velocity. If the product volume (“hold-up”) is increased by either increasing bed height or reactor diameter, or by decreasing granule velocity, polymer sticking tendency will increase. Thus, these constraints limit the maximum capacity of conventional solid phase polymerisation processes, which use one or more vertical cylindrical reactors.
Nowadays, growing polyester and PET demand has given rise to a need for solid-phase polymerisation processes by means of which it is possible to achieve a higher increase of polyester molecular weight and a higher production capacity, typically >300 metric tons/day on single plant.
The purpose of the present invention is therefore to provide a solid phase polymerisation process of polyester that allows to overcome the limitations of the processes known so far by permitting to achieve better results in term of increased intrinsic viscosity of the polyester.
A further purpose of the invention is therefore to provide a solid phase polymerisation process of polyester that allows to achieve higher production capacities.
In the solid phase polymerisation plants also the purge gas flow rate has to be just sufficient to effectively remove the reaction by-products. As a matter of fact, a gas excess results in higher costs both for its supply and for its regeneration and disposal.
Therefore, a further purpose of the invention is therefore to provide a solid phase polymerisation process of polyester that allows to reduce the costs due to the purge gas employment.
These and other purposes are achieved with the process according to the invention, as claimed in the attached claims.